Nanotube-based switching elements with multiple controls

ABSTRACT

Nanotube-based switching elements with multiple controls and circuits made from such. A switching element includes an input node, an output node, and a nanotube channel element having at least one electrically conductive nanotube. A control structure is disposed in relation to the nanotube channel element to controllably form and unform an electrically conductive channel between said input node and said output node. The output node is constructed and arranged so that channel formation is substantially unaffected by the electrical state of the output node. The control structure includes a control electrode and a release electrode, disposed on opposite sides of the nanotube channel element. The control and release may be used to form a differential input, or if the device is constructed appropriately to operate the circuit in a non-volatile manner. The switching elements may be arranged into logic circuits and latches having differential inputs and/or non-volatile behavior.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of and claims priority under 35 U.S.C.§ 120 to U.S. patent application Ser. No. 11/971,476, filed on Jan. 9,2008, entitled Nanotube-Based Switching Elements with Multiple Controls,which is a continuation of and claims priority under 35 U.S.C. §120 toU.S. patent application Ser. No. 11/197,196, filed on Aug. 4, 2005 nowU.S. Patent Publication No. 2005/0270824, entitled Nanotube-BasedSwitching Elements with Multiple Controls, which is a continuation ofand claims priority under 35 U.S.C. § 120 to U.S. patent applicationSer. No. 10/918,085, filed on Aug. 13, 2004, now U.S. Pat. No.6,990,009, entitled Nanotube-Based Switching Elements with MultipleControls, which claims priority under 35 U.S.C. § 119(e) to U.S.Provisional Patent Application No. 60/561,330, filed on Apr. 12, 2004,entitled Non-volatile CNT Dual-Rail Differential Logic, and also claimspriority under 35 U.S.C. § 119(e) to U.S. Provisional Patent ApplicationNo. 60/494,889, filed on Aug. 13, 2003, entitled NanoelectromechanicalNanotube-Based Logic, which are incorporated herein by reference intheir entirety.

This application is related to the following references:

-   U.S. Patent Application No. TBD, filed on date even herewith,    entitled Nanotube-Based Switching Elements with Multiple Controls    and Logic Circuits Having Said Elements;-   U.S. patent application Ser. No. 11/197,196, filed on Aug. 4, 2005,    now U.S. Patent Publication No. 2005/0035367, entitled    Nanotube-Based Switching Elements with Multiple Controls;-   U.S. patent application Ser. No. 10/917,794, filed on Aug. 13, 2004,    now U.S. Pat. No. 6,990,009, entitled Nanotube-Based Switching    Elements with Multiple Controls;-   U.S. patent application Ser. No. 10/917,893, filed on Aug. 13, 2004,    now U.S. Pat. No. 7,138,832, entitled Nanotube-Based Switching    Elements And Logic Circuits;-   U.S. patent application Ser. No. 10/917,606, filed on Aug. 13, 2004,    now U.S. Publication No. 2005/0035344, entitled Isolation Structure    for Deflectable Nanotube Elements; and-   U.S. patent application Ser. No. 10/918,181, filed on Aug. 13, 2004,    now U.S. Pat. No. 7,071,023, entitled Nanotube Device Structures and    Methods of Fabrication.

BACKGROUND

1. Technical Field

The present application generally relates to nanotube switching circuitsand in particular to nanotube switching circuits that use nanotubes toform a conductive channel of the switch and that may be interconnectedinto larger circuits, such as Boolean logic circuits.

2. Discussion of Related Art

Digital logic circuits are used in personal computers, portableelectronic devices such as personal organizers and calculators,electronic entertainment devices, and in control circuits forappliances, telephone switching systems, automobiles, aircraft and otheritems of manufacture. Early digital logic was constructed out ofdiscrete switching elements composed of individual bipolar transistors.With the invention of the bipolar integrated circuit, large numbers ofindividual switching elements could be combined on a single siliconsubstrate to create complete digital logic circuits such as inverters,NAND gates, NOR gates, flip-flops, adders, etc. However, the density ofbipolar digital integrated circuits is limited by their high powerconsumption and the ability of packaging technology to dissipate theheat produced while the circuits are operating. The availability ofmetal oxide semiconductor (“MOS”) integrated circuits using field effecttransistor (“FET”) switching elements significantly reduces the powerconsumption of digital logic and enables the construction of the highdensity, complex digital circuits used in current technology. Thedensity and operating speed of MOS digital circuits are still limited bythe need to dissipate the heat produced when the device is operating.

Digital logic integrated circuits constructed from bipolar or MOSdevices do not function correctly under conditions of high heat orextreme environments. Current digital integrated circuits are normallydesigned to operate at temperatures less than 100 degrees centigrade andfew operate at temperatures over 200 degrees centigrade. In conventionalintegrated circuits, the leakage current of the individual switchingelements in the “off” state increases rapidly with temperature. Asleakage current increases, the operating temperature of the devicerises, the power consumed by the circuit increases, and the difficultyof discriminating the off state from the on state reduces circuitreliability. Conventional digital logic circuits also short internallywhen subjected to certain extreme environments because electricalcurrents are generated inside the semiconductor material. It is possibleto manufacture integrated circuits with special devices and isolationtechniques so that they remain operational when exposed to suchenvironments, but the high cost of these devices limits theiravailability and practicality. In addition, such digital circuitsexhibit timing differences from their normal counterparts, requiringadditional design verification to add protection to an existing design.

Integrated circuits constructed from either bipolar or FET switchingelements are volatile. They only maintain their internal logical statewhile power is applied to the device. When power is removed, theinternal state is lost unless some type of non-volatile memory circuit,such as EEPROM (electrically erasable programmable read-only memory), isadded internal or external to the device to maintain the logical state.Even if non-volatile memory is utilized to maintain the logical state,additional circuitry is necessary to transfer the digital logic state tothe memory before power is lost, and to restore the state of theindividual logic circuits when power is restored to the device.Alternative solutions to avoid losing information in volatile digitalcircuits, such as battery backup, also add cost and complexity todigital designs.

Important characteristics for logic circuits in an electronic device arelow cost, high density, low power, and high speed. Conventional logicsolutions are limited to silicon substrates, but logic circuits built onother substrates would allow logic devices to be integrated directlyinto many manufactured products in a single step, further reducing cost.

Devices have been proposed which use nanoscopic wires, such assingle-walled carbon nanotubes, to form crossbar junctions to serve asmemory cells. (See WO 01/03208, Nanoscopic Wire-Based Devices, Arrays,and Methods of Their Manufacture; and Thomas Rueckes et al., “CarbonNanotube-Based Nonvolatile Random Access Memory for MolecularComputing,” Science, vol. 289, pp. 94-97, 7 Jul., 2000.) Hereinafterthese devices are called nanotube wire crossbar memories (NTWCMs). Underthese proposals, individual single-walled nanotube wires suspended overother wires define memory cells. Electrical signals are written to oneor both wires to cause them to physically attract or repel relative toone another. Each physical state (i.e., attracted or repelled wires)corresponds to an electrical state. Repelled wires are an open circuitjunction. Attracted wires are a closed state forming a rectifiedjunction. When electrical power is removed from the junction, the wiresretain their physical (and thus electrical) state thereby forming anon-volatile memory cell.

U.S. Patent Publication No. 2003-0021966 discloses, among other things,electromechanical circuits, such as memory cells, in which circuitsinclude a structure having electrically conductive traces and supportsextending from a surface of a substrate. Nanotube ribbons that canelectromechanically deform, or switch are suspended by the supports thatcross the electrically conductive traces. Each ribbon comprises one ormore nanotubes. The ribbons are typically formed from selectivelyremoving material from a layer or matted fabric of nanotubes.

For example, as disclosed in U.S. Patent Publication No. 2003-0021966, ananofabric may be patterned into ribbons, and the ribbons can be used asa component to create non-volatile electromechanical memory cells. Theribbon is electromechanically-deflectable in response to electricalstimulus of control traces and/or the ribbon. The deflected, physicalstate of the ribbon may be made to represent a corresponding informationstate. The deflected, physical state has non-volatile properties,meaning the ribbon retains its physical (and therefore informational)state even if power to the memory cell is removed. As explained in U.S.Patent Publication No. 2003-0124325, three-trace architectures may beused for electromechanical memory cells, in which the two of the tracesare electrodes to control the deflection of the ribbon.

The use of an electromechanical bi-stable device for digital informationstorage has also been suggested (c.f. U.S. Pat. No. 4,979,149:Non-volatile memory device including a micro-mechanical storageelement).

The creation and operation of bi-stable, nano-electro-mechanicalswitches based on carbon nanotubes (including mono-layers constructedthereof) and metal electrodes has been detailed in a previous patentapplication of Nantero, Inc. (U.S. Pat. Nos. 6,574,130, 6,643,165,6,706,402, 6,784,028, 6,835,591, 6,911,682, 6,919,592 and 6,924,538; andU.S. patent application Ser. Nos. 10/341,005, 10/341,055, 10/341,054,10/341,130 and 10/776,059, the contents of which are hereby incorporatedby reference in their entireties).

SUMMARY

The present invention provides nanotube-based switching elements withmultiple controls and circuits made from such.

Under one aspect of the invention, a switching element includes an inputnode, an output node, and a nanotube channel element having at least oneelectrically conductive nanotube. A control structure is disposed inrelation to the nanotube channel element to controllably form and unforman electrically conductive channel between said input node and saidoutput node. The output node is constructed and arranged so that channelformation is substantially unaffected by the electrical state of theoutput node.

Under another aspect of the invention, the control structure includes acontrol electrode and a release electrode, disposed on opposite sides ofthe nanotube channel element.

Under another aspect of the invention, channel formation is anon-volatile state.

Under another aspect of the invention, the control electrode and saidrelease electrode are arranged in relation to the nanotube channelelement to form and unform said conductive channel by causingelectromechanical deflection of said nanotube channel element.

Under another aspect of the invention, the nanotube channel elementincludes an isolation structure having two sets of electrodes disposedon opposite sides of the control structure, each set of electrodesincluding electrodes disposed on opposite side of the nanotube channelelement.

Under another aspect of the invention, the two sets of electrodes aresymmetrically disposed in relation to control structure.

Under another aspect of the invention, the nanotube channel element isin electrical communication with the input node and is positioned inspaced and crossed relation relative to the control electrode and therelease node and wherein deflection of said nanotube channel element isin response to electrostatic forces resulting from signals on the inputnode, the control electrode and the release node.

Under another aspect of the invention, deflection of the nanotubechannel element is in response to a differential signal relationshipapplied to the control electrode

BRIEF DESCRIPTION OF THE DRAWINGS

In the Drawing,

FIGS. 1A-1D illustrate cross-sectional views of a nanotube switchingelement of certain embodiments in two different states and include aplan view of such element;

FIGS. 2A-2C and 3A-3C show a schematic representation of a nanotubeswitching element;

FIG. 4 shows a schematic representation of an exemplary dual-rail input,dual-rail output inverter circuit constructed from nanotube switchingelements;

FIG. 5 shows a schematic representation of an exemplary dual-railflip-flop circuit constructed from nanotube switching elements;

FIGS. 6A-6B show a schematic representation of the use of an exemplarynanotube static ram cell and a dual-rail flip-flop circuit to constructa non-volatile ram cell;

FIG. 7 shows a schematic representation a dual rail nanotube-based NORcircuit; and

FIGS. 8A-8B shows an exemplary schematic representation of a volatile4-terminal nanotube switching device operated as a 3-terminal device ina latch circuit.

DETAILED DESCRIPTION

Preferred embodiments of the invention provide switching elements inwhich a nanotube-based channel may be controllably formed and unformed,so that a signal may be transferred from a signal node to an outputnode. The switching element includes multiple control electrodes tocontrol the formation and unformation of the channel, to provide adual-rail capability, and to be used in novel ways. The transferredsignal may be a varying signal or a reference signal, depending on themanner in which the switching element is utilized and arranged.Preferred embodiments provide an isolation structure so that such signaltransfer and the switching element's operation is substantiallyinvariant to the output state. For example, the output node may floatand/or be tied to other electrical components and the circuit willoperate in a predictable switch-like manner. Consequently, the switchingelements may be formed into larger circuits, such as Boolean logiccircuits. Under some embodiments, the switching elements are used ascomplimentary circuitry. Under some embodiments the switch maintains itsstate in the absence of power providing non-volatile logic. Under someembodiments the switching elements are used to form differential(dual-rail) logic.

FIG. 1A is a cross sectional view of a preferred nanotube switchingelement 100. Nanotube switching element includes a lower portion havingan insulating layer 117, control electrode 111, output electrodes 113c,d. Nanotube switching element further includes an upper portion havingrelease electrode 112, output electrodes 113 a,b, and signal electrodes114 a,b. A nanotube channel element 115 is positioned between and heldby the upper and lower portions.

Release electrode 112 is made of conductive material and is separatedfrom nanotube channel element 115 by an insulating material 119. Thechannel element 115 is separated from the facing surface of insulator119 by a gap height G102.

Output electrodes 113 a,b are made of conductive material and areseparated from nanotube channel element 115 by insulating material 119.

Output electrodes 113 c,d are likewise made of conductive material andare separated from nanotube channel element 115 by a gap height G103.Notice that the output electrodes 113 c,d are not covered by insulator.

Control electrode 111 is made of conductive material and is separatedfrom nanotube channel element 115 by an insulating layer (or film) 118.The channel element 115 is separated from the facing surface ofinsulator 118 by a gap height G104.

Signal electrodes 114 a,b each contact the nanotube channel element 115and can therefore supply whatever signal is on the signal electrode tothe channel element 115. This signal may be a fixed reference signal(e.g., Vdd or Ground) or varying (e.g., a Boolean discrete value signalthat can change). Only one of the electrodes 114 a,b need be connected,but both may be used to reduce effective resistance.

Nanotube channel element 115 is a lithographically-defined article madefrom a porous fabric of nanotubes (more below). It is electricallyconnected to signal electrodes 114 a,b. The electrodes 114 a,b andsupport 116 pinch or hold the channel element 115 at either end, and itis suspended in the middle in spaced relation to the output electrodes113 a-d and the control electrode 111 and release electrode 112. Thespaced relationship is defined by the gap heights G102-G104 identifiedabove. For certain embodiments, the length of the suspended portion ofchannel element 115 is about 300 to 350 nm.

Under certain embodiments the gaps G103, G104, G102 are in the range of5-30 nm. The dielectric on terminals 112, 111, and 113 a and 113 b arein the range of 5-30 nm, for example. The carbon nanotube fabric densityis approximately 10 nanotubes per 0.2×0.2 um area, for example. Thesuspended length of the nanotube channel element is in the range of 300to 350 nm, for example. The suspended length to gap ratio is about 5 to15 to 1 for non-volatile devices, and less than 5 for volatileoperation, for example.

FIG. 1B is a plan view or layout of nanotube switching element 100. Asshown in this figure, electrodes 113 b,d are electrically connected asdepicted by the notation ‘X’ and item 102. Likewise electrodes 113 a,care connected as depicted by the ‘X’. In preferred embodiments theelectrodes are further connected by connection 120. All of the outputelectrodes collectively form an output node 113 of the switching element100.

Under preferred embodiments, the nanotube switching element 100 of FIGS.1A and 1B operates as shown in FIGS. 1C and D. Specifically, nanotubeswitching element 100 is in an OPEN (OFF) state when nanotube channelelement is in position 122 of FIG. 1C. In such state, the channelelement 115 is drawn into mechanical contact with dielectric layer 119via electrostatic forces created by the potential difference betweenelectrode 112 and channel element 115. Output electrodes 113 a,b are inmechanical contact (but not electrical contact) with channel element115. Nanotube switching element 100 is in a CLOSED (ON) state whenchannel element 115 is elongated to position 124 as illustrated in FIG.1D. In such state, the channel element 115 is drawn into mechanicalcontact with dielectric layer 118 via electrostatic forces created bythe potential difference between electrode 111 and channel element 115.Output electrodes 113 c,d are in mechanical contact and electricalcontact with channel element 115 at regions 126. Consequently, whenchannel element 115 is in position 124, signal electrodes 114 a and 114b are electrically connected with output terminals 113 c,d via channelelement 115, and the signal on electrodes 114 a,b may be transferred viathe channel (including channel element 115) to the output electrodes 113c,d.

By properly tailoring the geometry of nanotube switching element 100,the nanotube switching element 100 may be made to behave as anon-volatile or a volatile switching element. By way of example, thedevice state of FIG. 1D may be made to be non-volatile by properselection of the length of the channel element relative to the gap G104.(The length and gap are two parameters in the restoring force of theelongated, deflected channel element 115.) Length to gap ratios ofgreater than 5 and less than 15 are preferred for non-volatile device;length to gap ratios of less than 5 are preferred for volatile devices.

The nanotube switching element 101 operates in the following way. Ifsignal electrode 114 and control electrode 111 (or 112) have a potentialdifference that is sufficiently large (via respective signals on theelectrodes), the relationship of signals will create an electrostaticforce that is sufficiently large to cause the suspended, nanotubechannel element 115 to deflect into mechanical contact with electrode111 (or 112). (This aspect of operation is described in the incorporatedpatent references.) This deflection is depicted in FIG. 1D (and 1C). Theattractive force streches and deflects the nanotube fabric of channelelement 115 until it contacts the insulated region 118 of the electrode111. The nanotube channel element is thereby strained, and there is arestoring tensil force, dependent on the geometrical relationship of thecircuit, among other things.

By using appropriate geometries of components, the switching element 100then attains the closed, conductive state of FIG. 1D in which thenanotube channel 115 mechanically contacts the control electrode 111 andalso output electrode 113 c,d. Since the control electrode 111 iscovered with insulator 118 any signal on electrode 114 is transferredfrom the electrode 114 to the output electrode 113 via the nanotubechannel element 115. The signal on electrode 114 may be a varyingsignal, a fixed signal, a reference signal, a power supply line, orground line. The channel formation is controlled via the signal appliedto the electrode 111 (or 112). Specifically the signal applied tocontrol electrode 111 needs to be sufficiently different in relation tothe signal on electrode 114 to create the electrostatic force to deflectthe nanotube channel element to cause the channel element 115 to deflectand to form the channel between electrode 114 and output electrode 113,such that switching element 100 is in the CLOSED (ON) state.

In contrast, if the relationship of signals on the electrode 114 andcontrol electrode 111 is insufficiently different, then the nanotubechannel element 115 is not deflected and no conductive channel is formedto the output electrode 113. Instead, the channel element 115 isattracted to and physically contacts the insulation layer on releaseelectrode 112. This OPEN (OFF) state is shown in FIG. 1C. The nanotubechannel element 115 has the signal from electrode 114 but this signal isnot transferred to the output node 113. Instead, the state of the outputnode 113 depends on whatever circuitry it is connected to and the stateof such circuitry. The state of output node 113 in this regard isindependent of channel element voltage from signal electrode 114 andnanotube channel element 115 when the switching element 100 is in theOPEN (OFF) state.

If the voltage difference between the control electrode 111 (or 112) andthe channel element 115 is removed, the channel element 115 returns tothe non-elongated state (see FIG. 1A) if the switching element 100 isdesigned to operate in the volatile mode, and the electrical connectionor path between the electrode 115 to the output node 113 is opened.

Preferably, if the switching element 100 is designed to operate in thenon-volatile mode, the channel element is not operated in a manner toattain the state of FIG. 1A. Instead, the electrodes 111 and 112 areexpected to be operated so that the channel element 115 will either bein the state of FIG. 1C or 1D.

The output node 113 is constructed to include an isolation structure inwhich the operation of the channel element 115 and thereby the formationof the channel is invariant to the state of the output node 113. Sincein the preferred embodiment the channel element is electromechanicallydeflectable in response to electrostatically attractive forces, afloating output node 113 in principle could have any potential.Consequently, the potential on an output node may be sufficientlydifferent in relation to the state of the channel element 115 that itwould cause deflection of the channel element 115 and disturb theoperation of the switching element 100 and its channel formation; thatis, the channel formation would depend on the state of an unknownfloating node. In the preferred embodiment this problem is addressedwith an output node that includes an isolation structure to prevent suchdisturbances from being caused.

Specifically, the nanotube channel element 115 is disposed between twooppositely disposed electrodes 113 b,d (and also 113 a,c) of equalpotential. Consequently, there are equal but opposing electrostaticforces that result from the voltage on the output node. Because of theequal and opposing electrostatic forces, the state of output node 113cannot cause the nanotube channel element 115 to deflect regardless ofthe voltages on output node 113 and nanotube channel element 115. Thus,the operation and formation of the channel is made invariant to thestate of the output node.

Under certain embodiments of the invention, the nanotube switchingelement 100 of FIG. 1A may be used as pull-up and pull-down devices toform power-efficient circuits. Unlike MOS and other forms of circuits,the pull-up and pull down devices may be identical devices and need nothave different sizes or materials. To facilitate the description of suchcircuits and to avoid the complexity of the layout and physical diagramsof FIGS. 1A-D, a schematic representation has been developed to depictthe switching elements.

FIG. 2A is a schematic representation of a nanotube switching element100 of FIG. 1A. The nodes use the same reference numerals. FIG. 2Aillustrates a non-volatile device in which the restoring mechanicalforce caused by nanotube elongation is insufficient to overcome van derWaals forces such that nanotube element 115 remains in a first or secondnon-volatile state even when voltage is removed. In a first non-volatilestate, nanotube switch 100 remains in the CLOSED (ON) state in contactwith control electrode 111 (shown in FIG. 1D) and output electrodes 113c and 113 d (shown in FIG. 1D) such that output electrode 113 is incontact with nanotube element 115, which in turn is in contact withsignal electrode 114. In a second non-volatile state, nanotube element115 remains in the OPEN (OFF) state in contact with release electrode112 such that nanotube element 115 is not in contact with outputelectrode 113 as illustrated in FIG. 1C when voltage is removed. FIG.2A′ is a schematic representation of a nanotube switching element 100 ofFIG. 1A. The nodes use the same reference numbers plus a prime (′) todistinguish FIG. 2A′ with respect to FIG. 2A. FIG. 2A′ illustrates avolatile device in which the restoring mechanical force caused bynanotube elongation is sufficient to disconnect nanotube element 115′from physical contact with control electrode 111′ and physical andelectrical contact with output electrode 113′ thus breaking the contactbetween signal electrode 114′ and output electrode 113′. The arrow 202is used to show the direction of the mechanical restoring force of thenanotube channel element 115′. For example, as depicted, the channelelement has a force away from control electrode 111′, i.e., if thechannel element 115′ were deflected into contact with electrode 111′amechanical restoring force would be in the direction of arrow 202. Anarrow indicating the direction of the mechanical restoring force is onlyused for devices designed to operate in the volatile mode.

FIGS. 2B-C depict a nanotube channel element 100 when used in a pull-uparrangement and its states of operation. For example, FIG. 2B is aschematic representation of the nanotube switching element in the OPEN(OFF) state illustrated in FIG. 1C, in which node 114 and the nanotubechannel element 115 are at V_(DD), the control electrode 111 is at apositive voltage, typically V_(DD), and the release electrode 112 is atzero volts. The nanotube channel element is not in electrical contactwith output node 113. FIG. 2C is a schematic representation of thenanotube switching element in the CLOSED (ON) state illustrated in FIG.1D. In this case, signal node 114 and the nanotube channel element 115are at V_(DD), the control electrode 111 is at zero volts, and therelease electrode 112 is at a positive voltage, typically V_(DD). Thenanotube channel element is deflected into mechanical and electricalcontact with the output node 113. Moreover, if as described above,geometries are selected appropriately, the contact will be non-volatileas a result of the Van der Waals forces between the channel element andthe control electrode. The state of electrical contact is depicted bythe short black line 204 representing the nanotube channel elementcontacting the output terminal 113. This results in the output node 113assuming the same signal (i.e., Vdd) as the nanotube channel element 115and signal node 114.

FIGS. 3A-C are analogous to those of FIGS. 2A-C, except they're used todepict a nanotube switching element 100 and its states when used as apull-down device.

In FIGS. 2 and 3, the nanotube switching element is always operated in away (at least when power is applied) where the control electrode 111 andthe release electrode 112 are always of opposite voltage values. If, forexample, control 111 is at zero volts, then release 112 is at a positivevoltage, typically V_(DD). If, however, control electrode 111 is at apositive voltage, typically V_(DD), then release electrode 112 is atzero volts. If a positive voltage is associated with a logic “1” state,and a zero voltage is associated with a logic “0” state, then logicstates applied to input and release are true and complement,respectively (or complement and true, respectively).

In this regard, the nanotube switching element 100 is operated as adual-rail differential logic element. Dual-rail differential logicdesign (or simply differential logic design) techniques applied to thenon-volatile 4-terminal nanotube switching devices 100 of FIG. 1 may beused to result in a non-volatile dual-rail differential logic family byusing such devices 100 to form fundamental building blocks for a logicfamily, e.g., NOT and NOR circuits. This non-volatile logic family willperform logic operations when activated, and preserve the logic state ina non-volatile mode when power is removed (or interrupted). This logicfamily may resume logic operation when power is restored, with eachlogic circuit in the same state as prior to power removal (orinterruption). Circuit examples follow.

FIG. 4 illustrates a dual rail inverter 420 according to certainembodiments of the invention. The inverter circuit 420 includes an upperportion 410T and a lower portion 410C.

The upper portion 410T includes a nanotube switching element 412arranged as a pull-up device with the signal node connected to Vdd, anda nanotube switching element 414 arranged as a pull-down device with thesignal node connected to ground. Both switching elements receive a trueversion A_(T) of logic signal A on their respective control nodes (111of FIG. 1A) via input link 411T, and both have their output nodes (113of FIG. 1A) connected together to output node 413T, which provides acomplement version A_(C) of logic signal A.

The lower portion 410C includes a nanotube switching element 416arranged as a pull-up device with the signal node connected to Vdd, anda nanotube switching element 418 arranged as a pull-down device with thesignal node connected to ground. Both switching elements receive acomplement version A_(C) of logic signal A on their respective controlnodes (111 of FIG. 1A) via input link 411C, and both have their outputnodes (113 of FIG. 1A) connected together to output node 413C, whichprovides true version A_(T) of logic signal A.

The input 411T of upper portion 410T is coupled to the release nodes ofboth switching elements of the lower portion 410C. Likewise, the input411C of lower portion 410C is coupled to the release nodes of bothswitching elements of the upper portion 410T.

Thus, inverter circuit 420 receives dual rail, differential inputs A_(T)and A_(C) of logic signal A, and provides corresponding inversions ofthe inputs on links 411 and 413, respectively, when operated asdescribed above. Moreover, the logic is non-volatile, meaning the gateretains its state even if power were interrupted from the circuit. Inaddition, because the circuit is arranged as a complementary logiccircuit with pull-up and pull-down devices, current flows (and power isconsumed) only during switching, so there is no DC current betweenV_(DD) and ground.

FIG. 5, using the same schematic notation as above, depicts anon-volatile state device 520 of certain embodiments. The latch 520 isformed by cross-connecting inverters 510 and 511. Output node 512 ofinverter 510 is connected to the control electrode inputs of inverter511 and the release electrode inputs of inverter 510. Output node 513 isconnected to the control electrode inputs of inverter 510 and therelease electrode inputs of inverter 511. State device 520 is anon-volatile storage element; that is, state device 520 will retain itslogic state if power is removed and assume the same state when power isrestored. State device 520 can be combined with logic gates (not shown)to generate a non-volatile NRAM storage cell as illustrated in FIG. 6A,and various types of flip flops and latches used in logic design such asa S-R, J-K, and other flip flop structures. FIG. 6B illustrates statedevices 520 used in a latch configuration. Current flows only duringswitching, so there is no DC current between V_(DD) and ground.

FIG. 6A illustrates a state device 620 used as a non-volatile nanotubestatic RAM cell (NRAM cell) 630 according to another embodiment of theinvention. In this embodiment, the control electrodes of the upperinverter 510A, and the release electrodes of the lower inverter 511A,are connected to a logical bit line (BL_(T)) input signal via selecttransistor 632 gated via signal WL and connected to state device 620 atnode 621. The control electrodes of the lower inverter, and the releaseelectrodes of the upper inverter, are connected to a complementaryversion (BL_(C)) of logical bit line (BL) input signal via selecttransistor 634 gated via signal WL and connected to state device 620node 622. In this fashion, a state device may be formed into anon-volatile NRAM storage cell 630. Inverters 510A and 511A are designedto be identical in electrical characteristics so that the NRAM cell isbalanced (does not favor one state over the other). Select devices 632and 634 are designed to be sufficiently large so as to supply sufficientdrive current to overcome the stored state of state device 620, as iswell known in the art of static RAM cell design.

NRAM cell 630 has the advantage of non-volatile storage. Also, thenanotube latch portion 620 may be formed using separate layers. Selecttransistors 632 and 634 are the only semiconductors required in the cellregion. The NRAM cell may be smaller than an SRAM cell because there isno need for a transistor latch. Eliminating the transistor flip-flopalso eliminates the need for both PMOS and NMOS transistors in cell, andboth P-Well and N-Well regions to accommodate the respective NMOS andPMOS transistor source and drain diffusions. A non-volatile NRAM cell630 may be smaller than a volatile (transistor-based) SRAM cell becauseonly NMOS select transistors 32 and 34 and contacts to nanotube basedlatch 620 layers are required. Current flows only during switching (cellwrite), or cell readout, so there is no DC current between V_(DD) andground. NRAM cell 630 is non-volatile; that is, the memory state ispreserved if power is turned-off (or disturbed) and resumes operation inthe same memory state as the memory state just prior to power turn-off(or interruption).

FIG. 6B depicts a state device 660 used as a non-volatile nanotube latch650 according to another embodiment of the invention. In thisembodiment, the control electrodes of the upper inverter 510B, and therelease electrodes of the lower inverter 511B, are connected to alogical input signal A_(T) (applied to IN1) via select transistor 662gated via clock signal CLK and connected to state device 660 at node671. The control electrodes of the lower inverter, and the releaseelectrodes of the upper inverter, are connected to a complementaryversion input signal A_(C) (applied to IN2) via select transistor 664gated via clock signal CLK and connected to state device 660 node 672.In this fashion, a state device may be formed into a nanotube latch 650.Inverter 511B is designed to be the dominant inverter (composed ofnanotube devices containing more nanotubes), and 510B is designed to bea feedback inverter (composed of nanotube devices containing lessnanotubes) that supplies the charge necessary to compensate fordischarge of a state device node in case of noise, for example, asdescribed in the reference book H. B. Bakoglu, “Circuits,Interconnections, and Packaging for VLSI”, Addison-Wesley PublishingCompany, pages 349-351. Inverters 511B, 510B, and pass transistors 662and 664 are ratioed to make sure that latch 660 will switch to thedesired state when data is being written to it. Feedback inverter 510Bhas to be sufficiently weak so that circuits (not shown) that drivestate device 660 through clocked devices 662 and 664 can overpowerfeedback inverter 510B and overcome the latch 650 state stored in statedevice 660 in the Bakoglu reference. Complementary logic latch outputsare labeled OUT1 and OUT2. Latch 650 is non-volatile; that is, the logicstate is preserved if power is turned-off (or disturbed) and resumesoperation in the same logic state as the logic state just prior to powerturn-off (or interruption).

FIG. 7 depicts a non-volatile, nanotube-based Cascode Voltage SwitchLogic (CVSL) circuit 700 according to certain embodiments. Circuit 700is composed of two complementary logic portions 740 and 742. The outputof logic circuit 700 is required to generate both true 720T andcomplement 720C logic outputs. Logic circuit 700 is non-volatile; thatis, the logic state is preserved if power is turned-off (or disturbed)and resumes operation in the same logic state as the logic state justprior to power turn-off (or interruption). Logic circuit 740 is anon-volatile nanotube-based NOR circuit, whose inputs are activated by Aand B in true and complement form, and whose corresponding output is(A_(T)+B_(T))_(C) as illustrated in FIG. 7. Logic circuit 742 is anon-volatile NAND circuit, whose output is A_(T)+B_(T), when activatedby inputs A_(C) and B_(C), and release A and B, as illustrated in FIG.7. Current flows only during switching, and there is no DC currentbetween V_(DD) and GND.

As discussed, the 4-terminal devices of FIG. 1 may also be constructedwith a nanotube length to gap size ratio of less than 5 to create avolatile device. This 4-terminal volatile device may also be operated asdual-rail, differential logic but will not preserve the logic state whenthe power to the circuit is interrupted. A schematic representation of avolatile 4-terminal device is shown in FIG. 2A′. A 4-terminal volatiledevice may be operated as a 3-terminal volatile device if the releaseelectrode is connected to the nanotube channel element through a lowresistance electrical path such as a metallization layer. Referring toFIG. 2A′, for example, release terminal 112′ is electrically connectedto nanotube signal electrode 114′. This allows single-rail volatilelogic, dual-rail volatile logic, and dual-rail non-volatile logic to bemixed on a single substrate using nanotube switching devices designedfor non-volatile operation, and nanotube switching devices designed forvolatile operation. FIGS. 8A-8B show examples of a 4-terminal deviceused as a 3-terminal device in a four device volatile state deviceconfiguration.

Nanotube-based logic may be used in conjunction with and in the absenceof diodes, resistors and transistors or as part of or a replacement toCMOS, biCMOS, bipolar and other transistor level technologies. Also, thenon-volatile flip flop may be substituted for an SRAM flip flop tocreate a NRAM cell. The interconnect wiring used to interconnect thenanotube device terminals may be conventional wiring such as AlCu, W, orCu wiring with appropriate insulating layers such as SiO2, polyimide,etc, or may be single or multi-wall nanotubes used for wiring.

The inventors envision additional configurations of volatile andnonvolatile or mixed nanoelectromechanical designs depending upon thespecific application, speed, power requirements and density desired.Additionally the inventors foresee the use of multiwalled carbonnanotubes or nanowires as the switching element of contact points withinthe switch. As the technology node decreases in size from 90 nm to 65 nmand below down to the size of individual nanotubes or nanowires theinventors foresee adapting the basic electromechanical switchingelements and their operation to a generation of nanoscale devices withscaleable performance characteristics concomitant with such sizereduction.

The nanotube switching element of preferred embodiments utilizesmultiple controls for the formation and unformation of the channel. Insome embodiments, the device is sized to create a non-volatile deviceand one of the electrodes may be used to form a channel and the othermay be used to unform a channel. The electrodes may be used asdifferential dual-rail inputs. Alternatively they may be set and used atdifferent times. For example, the control electrode may be used in theform of a clock signal, or the release electrode may be used as a formof clocking signal. Also, the control electrode and release electrodemay be placed at the same voltage, for example, such that the state ofthe nanotube cannot be disturbed by noise sources such as voltage spikeson adjacent wiring nodes.

A FIG. 1 device may be designed to operate as a volatile or non-volatiledevice. In the case of a volatile device, the mechanical restoring forcedue to nanotube elongation is stronger than the van der Waals retainingforce, and the nanotube mechanical contact with a control or releaseelectrode insulator is broken when the electrical field is removed.Typically, nanotube geometrical factors such as suspended length to gapratios of less than 5 to 1 are used for volatile devices. In the case ofa non-volatile device, the mechanical restoring force due to nanotubeelongation is weaker than the van der Waals retaining force, and thenanotube mechanical contact with a control or release electrodeinsulator remains un-broken when the electric field is removed.Typically, nanotube geometrical factors such as suspended length to gapratios of greater than 5 to 1 and less than 15 to 1 are used fornon-volatile devices. An applied electrical field generating anelectromechanical force is required to change the state of the nanotubedevice. Van der Waals forces between nanotubes and metals and insulatorsare a function of the material used in the fabrication nanotubeswitches. By way of example, these include insulators such as silicondioxide and silicon nitride, metals such as tungsten, aluminum, copper,nickel, palladium, and semiconductors such as silicon. For the samesurface area, forces will vary by less than 5% for some combinations ofmaterials, or may exceed 2× for other combinations of materials, so thatthe volatile and non-volatile operation is determined by geometricalfactors such as suspended length and gap dimensions and materialsselected. It is, however, possible to design devices by choosing bothgeometrical size and materials that exhibit stronger or weaker van derWaals forces. By way of example, nanotube suspended length and gapheight and fabric layer density, control electrode length, width, anddielectric layer thickness may be varied. Output electrode size andspacing to nanotube may be varied as well. Also, a layer specificallydesigned to increase van der Waals forces (not shown) may be addedduring the fabrication nanotube switching element 100 illustrated inFIG. 1. For example, a thin (5 to 10 nm, for example) layer of metal(not electrically connected), semiconductor (not electricallyconnected), or insulating material may be added (not shown) on theinsulator layer associated with control electrode 111 or releaseelectrode 112 that increases the van der Waals retaining force withoutsubstantial changes to device structure for better non-volatileoperation. In this way, both geometrical sizing and material selectionare used to optimize device operation, in this example to optimizenon-volatile operation.

In a complementary circuit such as an inverter using two nanotubeswitching elements 100 with connected output terminals, there can bemomentary current flow between power supply and ground in the invertercircuit as the inverter changes from one logic state to another logicstate. In CMOS, this occurs when both PFET and NFET are momentarily ON,both conducting during logic state transition and is sometimes referredto as “shoot-through” current. In the case of electromechanicalinverters, a momentary current may occur during change of logic state ifthe nanotube fabric of a first nanotube switch makes conductive contactwith the first output structure before the nanotube fabric of a secondnanotube switch releases conductive contact with the second outputstructure. If, however, the first nanotube switch breaks contact betweenthe first nanotube fabric and the first output electrode before thesecond nanotube switch makes contact between the second nanotube fabricand the second output electrode, then a break-before-make inverteroperation occurs and “shoot-through” current is minimized or eliminated.Electromechanical devices that favor break-before-make operation may bedesigned with different gap heights above and below the nanotubeswitching element, for example, such that forces exerted on the nanotubeswitching element by control and release electrodes are different;and/or travel distance for the nanotube switching element are differentin one direction than another; and/or materials are selected (and/oradded) to increase the van der Waals forces in one switching directionand weakening van der Waals forces in the opposite direction.

By way of example, nanotube switching element 100 illustrated in FIG. 1may be designed such that gap G102 is substantially smaller (50%smaller, for example) than gap G104. Also, gap G103 is made bigger suchthat nanotube element 115 contact is delayed when switching. Also,dielectric thicknesses and dielectric constants may be different suchthat for the same applied voltage differences, the electric fieldbetween release electrode 112 and nanotube element 115 is stronger thanthe electric field between control electrode 111 and nanotube element115, for example, to more quickly disconnect nanotube element 115 fromoutput terminals 113 c and 113 d. Output electrodes 113 c and 113 d maybe designed to have a small radius and therefore a smaller contact areain a region of contact with nanotube element 115 compared with the size(area) of contact between nanotube element 115 and the insulator oncontrol terminal 111 to facilitate release of contact between nanotubeelement 115 and output electrodes 113 c and 113 d. The material used forelectrodes 113 c and 113 d may be selected to have weaker van der Waalsforces respect to nanotube element 115 than the van der Waals forcesbetween nanotube element 115 and the insulator on release electrode 112,for example. These, and other approaches, may be used to design ananotube switching element that favors make-before-break operation thusminimizing or eliminating “shoot-through” current as circuits such asinverters switch from one logic state to another.

The material used in the fabrication of the electrodes and contacts usedin the nanotube switches is dependent upon the specific application,i.e. there is no specific metal necessary for the operation of thepresent invention.

Nanotubes can be functionalized with planar conjugated hydrocarbons suchas pyrenes which may then aid in enhancing the internal adhesion betweennanotubes within the ribbons. The surface of the nanotubes can bederivatized to create a more hydrophobic or hydrophilic environment topromote better adhesion of the nanotube fabric to the underlyingelectrode surface. Specifically, functionalization of a wafer/substratesurface involves “derivitizing” the surface of the substrate. Forexample, one could chemically convert a hydrophilic to hydrophobic stateor provide functional groups such as amines, carboxylic acids, thiols orsulphonates to alter the surface characteristics of the substrate.Functionalization may include the optional primary step of oxidizing orashing the substrate in oxygen plasma to remove carbon and otherimpurities from the substrate surface and to provide a uniformlyreactive, oxidized surface which is then reacted with a silane. One suchpolymer that may be used is 3-aminopropyltriethoxysilane (APTS). Thesubstrate surface may be derivitized prior to application of a nanotubefabric.

While single walled carbon nanotubes are preferred, multi-walled carbonnanotubes may be used. Also nanotubes may be used in conjunction withnanowires. Nanowires as mentioned herein is meant to mean singlenanowires, aggregates of non-woven nanowires, nanoclusters, nanowiresentangled with nanotubes comprising a nanofabric, mattes of nanowires,etc. The invention relates to the generation of nanoscopic conductiveelements used for any electronic application.

The following patent references refer to various techniques for creatingnanotube fabric articles and switches and are assigned to the assigneeof this application. Each is hereby incorporated by reference in theirentirety:

-   -   U.S. patent application Ser. No. 10/341,005, filed on Jan. 13,        2003, entitled Methods of Making Carbon Nanotube Films, Layers,        Fabrics, Ribbons, Elements and Articles;    -   U.S. patent application Ser. No. 09/915,093 (now U.S. Pat. No.        6,919,592), filed on Jul. 25, 2001, entitled Electromechanical        Memory Array Using Nanotube Ribbons and Method for Making Same;    -   U.S. patent application Ser. No. 10/033,032 (now U.S. Pat. No.        6,784,028), filed on Dec. 28, 2001, entitled Methods of Making        Electromechanical Three-Trace Junction Devices;    -   U.S. patent application Ser. No. 10/033,323 (now U.S. Pat. No.        6,911,682), filed on Dec. 28, 2001, entitled Electromechanical        Three-Trace Junction Devices;    -   U.S. patent application Ser. No. 10/128,117 (now U.S. Pat. No.        6,835,591), filed on Apr. 23, 2002, entitled Methods of NT Films        and Articles;    -   U.S. patent application Ser. No. 10/341,055, filed Jan. 13,        2003, entitled Methods of Using Thin Metal Layers to Make Carbon        Nanotube Films, Layers, Fabrics, Ribbons, Elements and Articles;    -   U.S. patent application Ser. No. 10/341,054, filed Jan. 13,        2003, entitled Methods of Using Pre-formed Nanotubes to Make        Carbon Nanotube Films, Layers, Fabrics, Ribbons, Elements and        Articles;    -   U.S. patent application Ser. No. 10/341,130, filed Jan. 13,        2003, entitled Carbon Nanotube Films, Layers, Fabrics, Ribbons,        Elements and Articles;    -   U.S. patent application Ser. No. 10/776,059, filed Feb. 11,        2004, entitled Devices Having Horizontally-Disposed Nanofabric        Articles and Methods of Making The Same; and    -   U.S. patent application Ser. No. 10/776,572 (now U.S. Pat. No.        6,924,538), filed Feb. 11, 2004, entitled Devices Having        Vertically-Disposed Nanofabric Articles and Methods of Making        the Same.

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The presentembodiments are therefore to be considered in respects as illustrativeand not restrictive, the scope of the invention being indicated by theappended claims rather than by the foregoing description, and allchanges which come within the meaning and range of the equivalency ofthe claims are therefore intended to be embraced therein.

1. A switching element comprising: an input node; an output node; ananotube channel element having at least one electrically conductivenanotube; a control structure disposed in relation to the nanotubechannel element to controllably form and unform an electricallyconductive channel between said input node and said output node, theelectrically conductive channel comprising said nanotube channelelement; wherein said output node is constructed and arranged so thatthe channel formation is substantially unaffected by the electricalstate of the output node; and wherein the electrically conductivechannel is volatilely formed.
 2. The switching element of claim 1wherein the control structure includes: a control electrode; a releaseelectrode; wherein the control electrode and release electrode aredisposed on opposite sides of and in spaced relation to the nanotubechannel element; and wherein in response to electrical stimulus at thecontrol electrode and release electrode, the control structurecontrollably forms and unforms said conductive channel by inducingelectromechanical deflection of said nanotube channel element.
 3. Theswitching element of claim 2 wherein the release electrode iselectrically coupled to the nanotube channel element.
 4. The switchingelement of claim 3 wherein the nanotube channel element is positionableto at least two positional states in response to a relationship ofsignals on the nanotube channel element and the control electrode; andwherein in the absence of signals on the nanotube channel element andthe control electrode, the nanotube element is positionable to at leastone positional state in response to a mechanical restoring forceresulting from the physical characteristics of the nanotube channelelement.
 5. The switching element of claim 2 wherein the releaseelectrode is electrically coupled to a reference voltage.
 6. Theswitching element of claim 2 wherein the release electrode iselectrically coupled to a ground voltage.
 7. The switching element ofclaim 2 wherein said nanotube channel element is in electricalcommunication with the input node.
 8. The switching element of claim 7wherein the nanotube channel element is positionable to at least twopositional states in response to a relationship of signals on thenanotube channel element, the control electrode and the releaseelectrode; and wherein in the absence of signals on the nanotube channelelement, the control electrode and the release electrode, the nanotubechannel element is positionable to at least one positional state inresponse to a mechanical restoring force resulting from the physicalcharacteristics of said nanotube channel element.
 9. The switchingelement of claim 8 wherein when the electrical stimulus applied to thecontrol electrode is of an approximately equivalent voltage as theelectrical stimulus applied to the release electrode, the positionalstate of the nanotube channel element is substantially unaffected byelectrical noise at the input terminal and the output terminal of thenanotube switching element.
 10. The switching element of claim 8 whereinforming a conductive channel comprises positioning the nanotube channelelement into a first of said at least two positional states wherein thenanotube channel element is in electrical contact with the output node11. The switching element of claim 10 wherein unforming a conductivechannel includes positioning the nanotube channel element into a secondof said at least two positional states wherein the nanotube channelelement is physically distant from the output node.
 12. The switchingelement of claim 8 wherein unforming a conductive channel includespositioning the nanotube channel element into said at least onepositional state in response to a mechanical restoring force wherein thenanotube channel element is physically distant from the output node. 13.The switching element of claim 2 wherein said output node includes anisolation structure disposed in relation to the nanotube channel elementso that channel formation is substantially invariant from the state ofthe output node.
 14. The switching element of claim 13 wherein theisolation structure comprises: an insulator disposed on the controlelectrode; a first spaced relation between the surface of said insulatorand the nanotube channel element in said at least one positional state;a second spaced relation between the release electrode and the nanotubechannel element in said at least one positional state; and two sets ofelectrodes disposed on opposite sides of the control structure, each setof electrodes including electrodes disposed on opposite sides of thenanotube channel element.
 15. The switching element of claim 13 whereinthe isolation structure comprises an insulator disposed on the releaseelectrode.
 16. The switching element of claim 14 wherein said firstspaced relation, said second spaced relation have different magnitudes.17. The switching element of claim 14 wherein the length of the nanotubechannel element is less than approximately five times the size of saidfirst spaced relation.
 18. The switching element of claim 14 constructedand arranged so that the mechanical restoring force of the nanotubechannel element exceeds the Van der Waals attraction force between thenanotube channel element and the adjacent surface of the controlstructure.